1. Field of the Invention
The present invention relates to a pressure sensor and a method of producing the pressure sensor, and more particularly to a pressure sensor including a flexible substrate and a method of producing such a pressure sensor. The pressure sensor according to the present invention can be suitably used, in particular, as a sensing portion of a manipulator, for example.
2. Discussion of the Related Art
Recent progress in the field of "mechatronics" or mechanoelectronics has required pressure sensors or force sensors to have characteristics not provided in the prior art.
For instance, a manipulator such as a robot arm requires a sensor for measuring in a real-time fashion a pressure of contact between the manipulator and an object body, for safe, adequate grasping or holding of the object body. Conventionally, piezoelectric sensors, semiconductor sensors or strain gages have been used as a sensor for such a manipulator.
The conventional pressure or force sensors as described above have a sensing element adapted to receive a pressure or force on a relatively hard flat surface thereof, and tend to suffer from stress concentration at a point of contact between the hard flat surface of the sensing element and the object body, where the object body to be handled has a spherical, oval or otherwise curved surface. Accordingly, the sensing element may mar the surface of the object body or damage the object body, particularly where the object is brittle.
In the light of the above drawback, there have been proposed various pressure sensors which are adapted, to protect the object body, such that the sensing element of the pressure sensor is in indirect surface contact with the object body. For example, the sensing element is covered by a soft or flexible material such as a sponge. Alternatively, a rubber material is bonded to the surface of the sensing element. An example of such pressure sensors is disclosed in JP-A-6-258153. However, these known pressure sensors wherein a force is applied to the sensing element through a flexible or elastic material have the following problems: i) low sensitivity or response to a change in the force or pressure; ii) low linearity of the relationship between the actual force or pressure and the sensor output, due to the use of a flexible or elastic material which generally has a high degree of hysteresis; and iii) inevitable increase in the size (thickness dimension) of the sensor.
There is known a computer system wherein a computer receives information from sensing means including sensors disposed on fingers of an operator's hand so that the motions of the fingers at their joints are detected. The conventional pressure sensors have an insufficient degree of flexibility, causing the operator to feel difficulty in freely moving the fingers or feel the fingers restricted by the pressure sensors.
To solve the problems indicated above, the pressure sensor requires to have a sensing element which is sufficiently flexible. In the light of this requirement, there have been proposed pressure sensors which use a film of a piezoelectric plastic material, more particularly, a PVDF film (polyvinylidene fluoride film) wherein a plastic material exhibits a piezoelectric effect, or a composite piezoelectric plastic film wherein fine particles of a piezoelectric ceramic material are dispersed in a matrix of a plastic material. Examples of such pressure sensors are disclosed in JP-B-51-49396, JP-A-5-318373. Although these pressure sensors have high flexibility, they suffer from relatively low energy conversion efficiency (electromechanical coupling factor), and relatively low heat resistance (highest permissible operating temperature as low as 80.degree. C.).
Thus, there has been a need of providing a pressure sensor which has high flexibility and an improved pressure or force sensing function, permitting accurate detection of a relatively small amount of change in a pressure or force which acts on object bodies of various shapes, like the hands or fingers of human beings.